This invention relates to vector corrected measurements of microwave circuits. More specifically, the invention provides a system for adjusting error factors, normally used to correct such measurements, to compensate for distortions due to imperfect assumptions of the value of reflection coefficients of impedance standards. Such imperfect assumptions may be caused, for example, by reactance in the measuring circuit due to variable positioning of circuit elements, such as probes, couplings, and the like.
Microwave measurements of very small planar circuits require highly accurate measurements of complex (magnitude and phase) reflection and transmission coefficients. The measurement system, whether used in a one-port or two-port mode, is subject to three major sources of repeatable errors correctable by complex error factors referred to as directivity (Ed), frequency response (Er), and source match (Es). The basic approach to determining and using such error factors is widely published, as exemplified by R. F. Bauer et al. "De-embedding and Unterminating", IEEE Trans. on MTT, Volume MTT-22, pages 282-288 (Mar. 1974), and J. Fitzpatrick, "Error Models For Systems Measurement," Microwave Journal (May 1978). It is well known that these three error factors Ed, Es, Er are mathematically related to the actual one-port reflection coefficient Sa and the measured one-port reflection coefficient Sm by the following equation (or variations thereof): ##EQU1## If the three error factors are known for the particular test frequency, the measured reflection coefficient Sm (magnitude and phase) can be corrected by solving the above equation for the actual reflection coefficient Sa.
In practice, values of the three error factors are conventionally determined by measuring the reflection coefficients (at the test frequencies of interest) of three independent primary impedance standards whose actual reflection coefficients are assumed to be known constants at all frequencies. Although different impedance standards may be used, the ones most commonly employed are the open-circuit, short-circuit, and load (termination) impedance standards whose actual reflection coefficients for purposes of calculating the error factors are assumed to be 1, -1, and 0, respectively (or with slight known offsets). The measured reflection coefficient Sm of the load standard is used to find Ed from the above equation. Thereafter, the equation can be solved simultaneously for the remaining two error factors Es and Er using the measurements Sm of the open and short standards, respectively.
The foregoing three assumed reflection coefficients of the impedance standards presume the absence of any unknown reactance affecting their reflection coefficients. However, it has been recognized that reactance does in fact affect such measurements and that the standards'reflection coefficients are therefore not completely known. In a technical paper by E. Strid, "Planar Impedance Standards and Accuracy Considerations in Vector Network Analysis" (June 1986), the effect of reactance on the assumed reflection coefficients of the foregoing impedance standards, and the resultant inaccuracies in error factor calculations, is discussed. Reactance affecting the measurements of the reflection coefficients of the open and short impedance standards is described as producing phase errors in the error factors, and thus phase errors in the ultimate corrected measurements of devices under test. On the other hand, reactance affecting the measurement of the reflection coefficient of the load impedance standard produces magnitude errors in the calculation of the error factors, and thus magnitude errors in the ultimate corrected measurements. However, the nature and values of the reactances, their variability with changes in position of a circuit element such as a probe, and the combined effects of two or more of these reactances have been difficult both to quantify and to interrelate. Accordingly, it has not previously been known how to adjust the error factors in a systematic or mutually compatible manner to compensate accurately for both magnitude and phase distortions caused by such reactances.